The first lines of this, one of Emily Dickinson's longest poems, are full of confused emotion. The speaker turns down a romantic proposal, not because they don't love the person in question, but because they love them too much. "I cannot live with You," the speaker tells this person, because "It would be Life –".
These first lines suggest that there's something both simple and overpowering about the speaker's love for the person they're speaking to. To live with them would mean nothing more nor less than life itself.
But the speaker doesn't feel they have access to life; life is something that's perpetually "over there" for them. If that's the case, they must be living something that isn't quite a life now—and, as the polyptoton on "live" and "life" points out, they expect to go on living that half-life in the future, without the beloved they "cannot live with." In other words, the speaker can't live with their beloved, and can't live without them! They're stuck in an awful limbo.
They get at the feeling of this choked-off predicament with an image of a tidy "Sexton"—a churchwarden, especially one in charge of a graveyard. This fellow, who raises shadows of both pious propriety and death, has a locked "Shelf," and "keeps the key" himself. The life the speaker and their beloved could live together isn't just locked up in that shelf, but "Behind" it, as if it's fallen down the back. One way or another, this life is out of reach and out of bounds.
The poem's shape mirrors the speaker's dilemma. These first few lines use an odd, jolting rhythm, alternating lines of iambic trimeter (that is, lines of three iambs, metrical feet with a da-DUM rhythm, as in "Behind") and iambic dimeter (lines of just two iambs). Listen to the first two lines:
I can- | not live | with You –
It would | be Life –
Readers who are familiar with sonnets or Shakespeare might find that this unusual meter feels oddly familiar: if these two lines were fused, they'd make one line of good old iambic pentameter (five iambs in a row, as in "I would | you were | as I | would have | you be"). Fittingly enough, it's as if a line from an old love poem has been lopped in two.
Similarly, notice the confusion in the poem's very shape here. The "Sexton" who keeps the keys to the "Shelf" only appears at the beginning of the next stanza. As readers will soon see, the way this poem uses enjambments that cross stanzas and line breaks means that there's more than one way to interpret these first words.